This invention relates to a method of effectively polarizing a thermoplastic resin film provided on both surfaces with metallic films as electrodes, and, particularly, to the method where at least one of the metallic films is divided to form a plurality of discrete electrode segments.
It is generally known that one can form a material having a high piezoelectricity and pyroelectricity by polarizing a thermoplastic resin having a high polarity, such as polyvinylidene fluoride, polyvinyl fluoride, polyvinyl chloride and nylon 11, etc. It is also known to use piezoelectricity, for example, for electric acoustic convertors, vibration measuring instruments, piezoelectric switches and pulsemeters or, the like, and to use the pyroelectricity, for example, for infrared ray sensors, temperature change measuring instruments, fire alarms and pyroelectric switches, etc.
Where such elements are used in electrical devices, a film shaped piezoelectric or pyroelectric element (referred to only as a piezoelectric element hereinafter) is used, and it is absolutely necessary to have electrodes adhering to both surfaces of the film. In the case where a considerably high electric field is applied across the two electrodes, e.g. when the thickness of the film is 6 to 50 microns and the voltage is more than 30 volts, creeping discharge is likely to occur on the side marginal portions, and the two electrodes tend to short-circuit on the terminal faces thereof so as to render the elements unusable. In order to avoid occurrence of such creeping discharge across the two electrodes, it is necessary to provide a marginal portion where no electrode is disposed on one or both side surfaces of the film, thereby electrically separating the electrodes on both surfaces.
It is extremely inefficient to separately manufacture a plurality of piezoelectric elements of various sizes. As a result it is preferable to polarize a film having a large surface area with metallic thin film electrodes adhered to both surfaces thereof and subsequently to cut off individual elements from the polarized film. In this case, each of the elements cut off from the large surface area film is covered by a metallic film even in the vicinity of the side marginal portions thereof. Where it is desirable to remove the marginal portions, for reasons mentioned above, this can be accomplished by dissolving them with means of a chemical agent. However such an operation is very complicated and troublesome. In order to eliminate this disadvantage, it is possible to provide electrode-free marginal portions on a film of a large surface area in advance, permit a plurality of electrodes to adhere on the remaining surface of the film, polarize each electroded section, and then cut off individual elements.
Furthermore, it has been proposed to manufacture film shaped keyboard switches and co-ordinate input systems employing the piezoelectric or pyroelectric elements. In such cases, in order for the changes in piezoelectricity or pyroelectricity produced on each co-ordinate of the key to constitute a separate input, the electrode on one or both surfaces of the film must comprise a plurality of spot-like or linear individually independent electrodes.
As mentioned hereinabove, where a film is polarized which has a plurality of independent electrodes adhered on one surface or both surfaces thereof, it is necessary to apply voltage to the individually independent electrodes. It is quite complicated, however, to make an electrical connection to each of the individually independent electrodes. Thus, in order to apply voltage to each electrode in the most simple manner, it is preferable to provide electrodes of a large enough area to "face-contact" with some or all of the plurality of independent electrodes and to apply a voltage via the large area electrode into the small local electrodes. However, when electrodes of such a large surface area are used as a power source, the following problem occurs.
Stated in brief, in case of manufacturing piezoelectric elements etc., it is known that if other conditions are the same, in general the more the applied voltage increases, the higher piezoelectricity and pyroelectricity become. Further, it is also known that, within the range where the nature of the film (particularly, crystal form and the degree of crystallization) does not change so much, generally speaking, the more polarization temperature increases, the higher the piezoelectricity and pyroelectricity become. However, in general, the insulation resistance of the film will drop with a temperature rise, and the strength of the electric field to be formed thereby will decrease so that it is absolutely necessary to select an optimum condition in the light of the relationship between the applied voltage and the polarization temperature.
Anyhow, where it is desired to obtain a film having a high piezoelectricity or pyroelectricity, it is necessary to polarize the film by applying thereto a high voltage which is near the breakdown voltage for the polarization temperature involved. However, it is difficult to bring electrodes comprised of, for example, copper or iron plates, into tight contact with a thin film, and so there is a tendency for several air gaps to be formed in places thereon. Since the breakdown voltage of air is lower than that of the film to be polarized, during polarization corona discharge tends to occur in the air gap portions. Therefore, the possibility of pin holes being formed on the thin film exists. Further, when a voltage is applied across the thin film electrodes adhered on both surfaces of a thermoplastic resin film, even if the applied voltage exceeds the breakdown voltage of the film thereby causing a short-circuit, if the electrodes are thin, the electrodes formed in the vicinity of the short-circuited portion will evaporate to recover the insulation resistance between the two electrodes, resulting in a self-recovery action so that the film can be continuously polarized without causing serious damage. However, in the case where the metal film is too thick to be removed by evaporation, such self-recovery action can not be expected, and so it becomes impossible to effect continuously polarization of the film after the short-circuit takes place.